Unlikely hero John O'Shea gave Manchester United a priceless lead to take into the second leg of their Champions League semi-final with Arsenal.

The defender's nerveless 17th-minute effort was the least Sir Alex Ferguson's side deserved for a near faultless display, in which Manuel Almunia made a quartet of stunning saves and Cristiano Ronaldo rattled the Gunners' crossbar.

In addition, Ryan Giggs had a goal controversially disallowed for offside on his 800th appearance, meaning Arsenal headed home glad to still have a chance of reaching Rome on May 27.

However, having indicated yesterday a single-goal victory and a clean sheet would represent a perfect result, Ferguson got exactly what he wanted and his side must be favourites to get through.

A game of pure football had been expected after the sterile affair in Catalonia last night. The problem for Arsenal was only United produced it.

The first half-hour was as one-sided as a game at this level is possible to be.

With screening midfielders Alexandre Song and Abou Diaby failing completely to quell the tide of onrushing home players, Wenger's team were almost submerged before they had managed so much as a shot.

United would have been in front with only a minute on the clock had Almunia not managed to claw away a looping Wayne Rooney header that seemed destined to drop in.

It proved to be the sign of things to come for Arsenal, who were pulled apart at regular intervals, with Almunia resembling King Canute trying to hold back the tide.

The Spaniard pulled off two splendid stops in the space of a minute to deny Ronaldo.

His first was the pick, parrying a firm header from point-blank range after Carlos Tevez had crossed. A curling shot from the boot of the Ballon D'Or winner followed shortly afterwards which Almunia was also equal to.

It would be too simplistic to say Tevez was the catalyst for such relentless attacking.

However, it is undeniable the Argentinian triggered Saturday's amazing comeback against Tottenham and, surprisingly selected ahead of record signing Dimitar Berbatov, Tevez kept going exactly the same way.

It was a close-range effort from Tevez, saved by Almunia, that proved to be the prelude to United's opener.

Anderson's corner fell kindly for Michael Carrick at the far post, with the England star electing for a firmly driven cross into the six-yard box.

Against his former club, Mikael Silvestre was eager to make a positive impression. That did not include deflecting the ball to O'Shea.

Leaning back, the Irishman crashed his shot into the roof of Almunia's net.

O'Shea has form for that kind of thing. A couple of years ago, he scored a last-minute winner at Anfield and then smacked one in at Everton in what turned out to be a title-winning victory.

If Arsenal thought the second half at least gave them an opportunity to enjoy a more even share of possession and chances, they were mistaken.

A rocket shot from Ronaldo cannoned away for a corner off Silvestre, the same player also sliding in to clear a low Evra cross bound for Tevez.

Anderson blazed over from the edge of the area, which was perhaps no surprise given he has not scored for United in almost two years since arriving from FC Porto. Under the circumstances, it was probably just as well an offside flag had earlier been incorrectly raised against the Brazilian as he threatened to charge clear after collecting an O'Shea pass.

Having given so much, Tevez was not amused to be replaced by Berbatov midway through the second-half.

Not that a capacity crowd had much time to consider his reaction after a loud rendition of 'Argentina' that followed 'sign him up', a cry seemingly destined to go unheeded.

Attention was diverted by the thunderous 25-yard effort from Ronaldo that smacked into the crossbar as United strained every sinew in their search for a second.

In response, Cesc Fabregas was off target with a couple of half-chances, Emmanuel Adebayor's volley flew over and substitute Nicolas Bendtner might well have equalised with a header from the free-kick that caused the collision which forced Rio Ferdinand off with what seemed like a rib injury.

The sight of Ferdinand trying to get back on even though Jonny Evans had already replaced him, emphasised how much this means to United.

Arsenal will need to exceed that determination if they are to overhaul the deficit next week.

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